Looking for a new lens?

The sensors in the latest cameras actually have better resolving power than all but the best L series lenses by Canon. So, the technical answer is yes, your lens is limiting your photography. However, the real question is: do you notice it? If your answer is yes, then get one of the Canon L series lenses in a shorter focal length (for the landscapes). You can't do better than that.

Now I must direct your attention to what you think is inferior about your images that you would want to blame on your lens. I don't have the 7D. I do hear, however, that she is something of a high strung thoroughbred in the autofocus area. Users who are not ready for this camera have complained that they think that something's wrong with it because of out-of-focus images. In most cases, the cause is user inexperience. So I would suggest getting a little more experience with it before deciding to drop a chunk of change on an expensive lens.

Yes you could get better images with better lenses. But since you are shooting landscapes (and using a tripod of course) you should be using the optimum or near optimum apertures for your lens. (f8-f11)

If you want to see how your lens stacks up compared to other lenses then look at http://www.the-digital-picture.com/Reviews/Tamron-... and go to the ISO 12233 Crop page. Here you can do a resolution compare at different focal lengths and apertures between two selected lenses. Move your mouse over the image to see the right selected lens.

Some lenses are only tested with a full frame body but you still will get useful information.

Your lens seems to do better at some focal lenths than others. This is not a surprise. Expected actually.